The invention relates to a method of coating a running material web wherein the coating material is applied as a free-falling curtain to the web and the edges of the curtain are split off by splitters at both sides for setting the coating width before contact with the material web. In addition the invention concerns a coating device that is particularly suited for carrying out the method according to the invention and to a splitter particularly usable in the coating device.
In order to coat a running material web, for instance of paper, cardboard, plastic, or metal, so-called curtain coaters are used where the coating material (pigment, plastic dispersion, etc.) is applied from a slot nozzle in a free-falling curtain to the material web. In order to produce a stable and uniform curtain across the coating width, WO 98/47630 and DE 197 35 588 describe how the two curtain edges are guided by respective guides extending downward from the slot nozzle to near the material web. In order to set the desired curtain width, the curtain edges are split off before the curtain actually contacts the material web by splitters that extend transverse to the web-travel direction underneath the guides. The two splitters each have an upwardly directed splitting edge and an outwardly directed inclined upper catching surface on which the material split from the edge of the curtain can flow outward.
If the curtain coater is used to apply dispersion adhesives to a web-shaped splitter in the production of self-sticking labels, release strips, etc. or to apply a barrier layer in the production of a water-impermeable packing material, it is necessary to be able to set the coating width over a wide range since the web width varies considerably depending on the material being coated.
It is therefore an object of the invention to improve on a method of and apparatus of the described type such that widely varying coating widths can be set without diminishing the coating quality.
A further object of the invention is to provide a splitter that is particularly suited for use in the coating apparatus according to the invention.
This object is attained in that first a first outer splitter at each edge creates a narrow curtain and then a second inner splitter at each edge creates the desired coating width.
The provision of two splitters on each side makes it possible to set a very narrow coating width by splitting off a substantial lateral region of the curtain without coating errors on the longitudinal edges. In order to avoid coating errors (variations in the application density or the coating width, gaps in the curtain, etc.) inner splitters are used whose splitter edges are very short. The bulk of the flow of coating material is split off by the outer splitters whose edges are sufficiently high that they avoid an overflow of the split-off coating material. The inner splitters take care of the rest of the curtain edge. They can thus have a lower height without the split-off coating material flowing inward over the splitting edge as is not wanted. A shorter height of the splitting edge of the inner element is advantageous since its inner edge extending parallel to the flow direction of the curtain also serves to guide the curtain edge. Too great a height allows separation of the curtain edge from the inner face too far above the web being coated. In this case its position moves and a ridge is formed of the coating material on the material web.